Process for producing carboxylic acid and system for producing the same

ABSTRACT

In the presence of a catalytic system, an alcohol having “n” carbon atom(s) or a derivative thereof is allowed to react with carbon monoxide in a reactor  3  continuously, a higher bp catalyst component is separated from the resultant reaction mixture by a catalyst-separating column  5  to give a crude mixture, the crude mixture is fed to a higher bp component-separation column  8  to separate an overhead fraction from a bottom fraction containing at least a carboxylic acid having “n+2” carbon atoms, the overhead fraction is fed to a carboxylic acid-separating column  11 , and are distilled in the presence of at least water and an ester of the carboxylic acid with the alcohol to separate a overhead fraction containing at least the ester and water from a bottom fraction containing the carboxylic acid having “n+1” carbon atoms. The overhead fraction from the carboxylic acid-separating column is fed to an aldehyde-separating column  14  to remove an overhead fraction containing an aldehyde. Such a process insures that impurities are efficiently separated from a reaction mixture by carbonylation of an alcohol, and that a carboxylic acid is purified easily at a lower cost.

TECHNICAL FIELD

The present invention relates to a process for industrially producing acarboxylic acid such as acetic acid, in particular a process forproducing a carboxylic acid by a carbonylation reaction of an alcohol(such as methanol) or a derivative thereof, and to a system forproducing the same.

BACKGROUND ART

Carboxylic acids, notably acetic acid, have been used as an ingredientof acetic acid ester compounds, acetic anhydride, terephthalic acid orothers, and are one of basic chemicals being in heavy usage in variousfields such as the petrochemical industry, the organic synthesisindustry, the pharmaceutical and agricultural chemical industry, or theindustry of polymer chemistry.

As a process for industrially producing acetic acid, various methodssuch as oxidation of acetaldehyde and direct oxidation of a hydrocarbon(e.g., petroleum naphtha, butane) have been known. Among others, amethod currently universally adopted for industrially producing aceticacid is a method of producing acetic acid by continuously allowingmethanol to react with carbon monoxide to carbonylate methanol [JapanesePatent Publication No. 3334/1972 (JP-47-3334B)].

Regarding the foregoing production method of acetic acid bycarbonylation of methanol, New Petrochemical Process (Japan PetroleumInstitute) p. 316 (1986) describes purification of acetic acid by thefollowing four distillation steps (1) to (4):

-   -   (1) in a lower-boiling component-separation column, separating a        fraction having a lower-boiling point through the overhead of        the column, and in parallel separating a higher boiling point        fraction containing a catalyst through the bottom of the column,        and returning the bottom fraction to a reactor,    -   (2) in a dehydration column, separating moisture not removed by        the lower-boiling component-separation column through the        overhead of the dehydration column, and returning the moisture        to the reactor,    -   (3) in a distillation column for obtaining acetic acid,        separating propionic acid being a component having a        higher-boiling point through the bottom of the distillation        column, and    -   (4) in a purification column, separating slight amounts of a        lower-boiling point fraction and higher-boiling point fraction        through the overhead and bottom of the purification column,        respectively.

In a binary system of acetic acid and water, however, it is difficult toseparate acetic acid from water because of a low relative volatilitybetween water and acetic acid based on the relation of vapor-liquidequilibrium, and therefore, it is necessary to increase the number ofplates or to enhance the reflux ratio of a distillation column in orderto separate acetic acid from water efficiently. In particular,industrial production of acetic acid needs removal of water from a crudereaction solution in a purification step. However, it is difficult toseparate of water from acetic acid so that equipment expenses and energycost are significantly increased due to increase of the number of platesor enhancement of the reflux ratio.

Moreover, in the case producing acetic acid by carbonylation ofmethanol, since the reaction needs water, the crude reaction solutioncontains water. In order to obtain acetic acid as a finished product,water must be removed so that the moisture content becomes not more thana given concentration. Generally, as described above, water is removedwith the use of the dehydration column. However, an excess amount ofacetic acid is distilled off along with water, and the resultant mixtureof acetic acid and water is recycled to a reactor. Such a method causessignificantly large energy loss because the excess amount of acetic acidcirculates through the system.

Japanese Patent Publication No. 30093/1982 (JP-57-30093B) proposes amethod for separating acetic acid which comprises adding methyl acetateas a third component in a dehydration column, and allowing methylacetate to azeotrope with water. In order to add the third component,however, extra equipment and control are necessary, in addition there isalso a possibility that the third component is contaminated in aceticacid as a finished product.

Moreover, acetaldehyde and/or propionic acid are contained in a reactionmixture obtained by carbonylation of methanol. Acetaldehyde in itselfcauses impairment of quality of acetic acid. In addition, circulation ofacetaldehyde through the system not only results in concentration butalso forms a compound having a higher-boiling point, and higher-boilingimpurities are generated, whose boiling point is close to that of aceticacid as the finished product. The contamination of the impurities in theproduct further causes deterioration in quality of the finished product.Moreover, the contamination of the foregoing propionic acid in aceticacid as the finished product effects deterioration in quality of thesubsequent product.

Regarding a method for removing a halide contained in a carboxylic acidin a ppb order, Japanese Patent Application Laid-Open No. 5367/1971(JP-46-5367B) discloses that a halogen-free carboxylic acid is obtainedby removing higher-boiling impurities in a first distillation column andremoving lower-boiling impurities containing a halide in a seconddistillation column to purify a carboxylic acid as a product. However,this literature fails to disclose a concrete process for purifying acrude reaction solution containing additional other impurities inaddition to a halide.

It is therefore an object of the present invention to provide a processand system for efficiently separating impurities from a reaction mixtureobtained by a carbonylation reaction of an alcohol (in particularmethanol) to produce a purified carboxylic acid (in particular aceticacid) easily and efficiently.

It is another object of the present invention to provide a process andsystem which insures production of a carboxylic acid (i.e., a purifiedcarboxylic acid) with removing water without circulating an excessamount of a carboxylic acid (in particular acetic acid) through areaction system.

It is still another object of the present invention to provide a processand system which insures production of a highly purified carboxylic acid(in particular acetic acid) without addition of an azeotropic component.

It is a further object of the present invention to provide a process andsystem which insures production of a highly purified carboxylic acid (inparticular acetic acid) at high energy efficiency.

DISCLOSURE OF INVENTION

The inventors of the present invention made intensive studies to achievethe above objects and finally found that beforehand removal (orelimination) of a bottom (or higher boiling point) fraction (e.g., acarboxylic acid having “n+2” carbon atoms, a higher-boiling pointcatalytic component) from a reaction product obtained by carbonylationof an alcohol having “n” carbon atom(s) insures utilization of water andan ester of the alcohol with a carboxylic acid having “n+1” carbon atomsgenerated in the reaction system, as azeotropic solvents, efficientpurification of the carboxylic acid having “n+1” carbon atoms at a highenergy efficiency, and thereby reducing significantly the productioncost. The present invention was accomplished based on the abovefindings.

That is, the present invention includes a process for producing acarboxylic acid comprising

-   -   allowing an alcohol having “n” carbon atom(s) or a derivative        thereof to react with carbon monoxide continuously in the        presence of a catalytic system, and purifying the resultant        reaction mixture to give a purified carboxylic acid having “n+1”        carbon atoms, wherein a higher-boiling point (or higher bp)        catalyst component is separated from the reaction mixture to        give a crude mixture containing at least a carboxylic acid        having “n+2” carbon atoms, a carboxylic acid having “n+1” carbon        atoms, an ester of the carboxylic acid having “n+1” carbon atoms        with the alcohol, and water; the crude mixture is fed to a        higher-boiling point (or higher bp) component-separation column,        and is separated into a bottom fraction and an overhead        fraction, the bottom fraction contains at least the carboxylic        acid having “n+2” carbon atoms, and the overhead fraction        contains at least the carboxylic acid having “n+1” carbon atoms,        the ester of the carboxylic acid having “n+1” carbon atoms with        the alcohol, and water; and the overhead fraction from the        higher bp component-separation column is separated by a        carboxylic acid-separating column into a bottom fraction and an        overhead fraction, the bottom fraction contains the carboxylic        acid having “n+1” carbon atoms, and the overhead fraction        contains at least the ester and water. The reaction mixture may        contain water in a proportion of not more than 20% by weight.

In the production process of the present invention, the crude mixturemay further contain an aldehyde having “n+1” carbon atoms, and the crudemixture may be fed to the higher bp component-separation column. In theproduction process, the crude mixture containing the carboxylic acidhaving “n+2” carbon atoms, an aldehyde having “n+1” carbon atoms, thecarboxylic acid having “n+1” carbon atoms, the ester of the carboxylicacid having “n+1” carbon atoms with the alcohol and water may be fed tothe higher bp component-separation column, and may be separated into thebottom fraction and the overhead fraction, the bottom fraction containsthe carboxylic acid having “n+2” carbon atoms, and the overhead fractioncontains the aldehyde having “n+1” carbon atoms, the carboxylic acidhaving “n+1” carbon atoms, the ester of the carboxylic acid having “n+1”carbon atoms with the alcohol, and water; the overhead fraction from thehigher bp component-separation column may be separated by the carboxylicacid-separating column into the bottom fraction and the overheadfraction, the bottom fraction contains the carboxylic acid having “n+1”carbon atoms, and the overhead fraction contains at least the aldehyde,the ester and water; the overhead fraction from the carboxylicacid-separating column may be separated by an aldehyde-separating columninto an overhead fraction and a bottom fraction, the overhead fractioncontains the aldehyde, and the bottom fraction contains at least theester and water; and the bottom fraction from the aldehyde-separatingcolumn may be recycled to the reaction system.

The catalytic system may comprise a catalyst containing a metal elementof the Group 8 of the Periodic Table of Elements, and an alkyl halide(and if necessary an alkali metal halide); distillation in thecarboxylic acid-separating column may be carried out in the presence ofthe ester of the carboxylic acid having “n+1” carbon atoms with thealcohol, the alkyl halide and water for separating the bottom fractionfrom the overhead fraction, the bottom fraction contains the carboxylicacid having “n+1” carbon atoms, and the overhead fraction containswater, the alkyl halide and the ester; the overhead fraction from thecarboxylic acid-separating column may be separated by thealdehyde-separating column into the overhead fraction and the bottomfraction, the overhead fraction contains the aldehyde, and the bottomfraction contains water, the alkyl halide and the ester; and the bottomfraction from the aldehyde-separating column may be recycled to thereaction system.

Moreover, in the production process of the present invention, the crudemixture in which at least an aldehyde having “n+1” carbon atoms has beenremoved may be fed to the higher bp component-separation column. Thehigher bp catalyst component may be separated from the reaction mixtureto give a crude mixture, and the resultant crude mixture may be fed to alower-boiling point (or lower bp) component-separation column, and maybe separated into the overhead fraction and the bottom fraction, theoverhead fraction contains at least an aldehyde having “n” carbonatom(s), and the bottom fraction contains at least the carboxylic acidhaving “n+2” carbon atoms; the bottom fraction from the lower bpcomponent-separation column may be separated by the higher bpcomponent-separation column into the bottom fraction and the overheadfraction, the bottom fraction contains the carboxylic acid having “n+2”carbon atoms, and the overhead fraction contains at least the carboxylicacid having “n+1” carbon atoms, the ester of the carboxylic acid having“n+1” carbon atoms with the alcohol, and water; and the overheadfraction from the higher bp component-separation column may be separatedby the carboxylic acid-separating column into the bottom fractioncontaining the carboxylic acid having “n+1” carbon atoms and theoverhead fraction containing at least the ester and water. Distillationin the carboxylic acid-separating column may be carried out in thepresence of at least the ester and water to separate the bottom fractionfrom the overhead fraction.

The catalytic system may comprise a catalyst containing a metal elementof the Group 8 of the Periodic Table of Elements, and an alkyl halide(and if necessary an alkali metal halide); distillation in thecarboxylic acid-separating column may be carried out in the presence ofthe ester, the alkyl halide and water to give the bottom fractioncontaining the carboxylic acid having “n+1” carbon atoms, and theoverhead fraction containing at least the ester, the alkyl halide andwater.

The overhead fraction separated by the carboxylic acid-separating columnmay be recycled to the reaction system. Moreover, the overhead fractionseparated by the lower bp component-separation column may be further fedto an aldehyde-separating column to separate an overhead fractioncontaining an aldehyde having “n+1” carbon atoms to give a bottomfraction containing at least the ester and water; and the bottomfraction may be recycled to the reaction system.

According to the present invention, in a process which comprisesallowing at least one member selected from the group consisting ofmethanol, methyl acetate and dimethyl ether to react with carbonmonoxide continuously in the presence of the catalytic system, andpurifying the resultant reaction mixture to produce a purified aceticacid, the higher bp catalyst component may be separated from thereaction mixture to give the crude mixture; the crude mixture may be fedto the higher bp component-separation column, and may be separated intothe bottom fraction and the overhead fraction, the bottom fractioncontains at least propionic acid, and the overhead fraction contains atleast acetic acid, methyl acetate and water; and the overhead fractionfrom the higher bp component-separation column may be fed to thecarboxylic acid-separating column to distill the fraction in thepresence of at least the methyl acetate, and may be separated into thebottom fraction and the overhead fraction, the bottom fraction containsthe acetic acid, and the overhead fraction contains at least the methylacetate and water.

The catalytic system may comprise a catalyst containing a rhodiumcatalyst, an alkali metal iodide and methyl iodide; the crude mixturemay be separated by the higher bp component-separation column into thebottom fraction and the overhead fraction, the bottom fraction containsat least propionic acid, and the overhead fraction contains acetic acid,methyl acetate, methyl iodide and water; and the overhead fraction fromthe higher bp component-separation column may be distilled by thecarboxylic acid-separating column in the presence of the methyl acetateand methyl iodide, and may be separated into the bottom fraction and theoverhead fraction, the bottom fraction contains the acetic acid, and theoverhead fraction contains the methyl acetate, methyl iodide and water.

The present invention also discloses a system corresponding to theabove-mentioned production process. That is, the production system ofthe present invention comprises a reaction system for allowing analcohol having “n” carbon atom(s) or a derivative thereof to react withcarbon monoxide continuously in the presence of a catalytic system; acatalyst-separating column for separating a higher bp catalyst componentfrom a reaction mixture generated in the reaction system; a higher bpcomponent-separation column for separating a crude mixture obtained by aseparation in the catalyst-separating column and containing at least acarboxylic acid having “n+2” carbon atoms, a carboxylic acid having“n+1” carbon atoms, an ester of the carboxylic acid having “n+1” carbonatoms with the alcohol, and water, into a bottom fraction and anoverhead fraction, wherein the bottom fraction contains at least thecarboxylic acid having “n+2” carbon atoms, and the overhead fractioncontains at least the carboxylic acid having “n+1” carbon atoms, theester of the carboxylic acid having “n+1” carbon atoms with the alcohol,and water; and a carboxylic acid-separating column for separating theoverhead fraction separated by the higher bp component-separation columninto a bottom fraction and an overhead fraction, wherein the bottomfraction contains the carboxylic acid having “n+1” carbon atoms, and theoverhead fraction contains at least the ester and water.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a flow diagram for illustrating an embodiment of aproduction (purification) process of a carboxylic acid of the presentinvention.

FIG. 2 shows a flow diagram for illustrating another embodiment of aproduction (purification) process of a carboxylic acid of the presentinvention.

FIG. 3 shows a flow diagram for illustrating still another embodiment ofa production (purification) process of a carboxylic acid of the presentinvention.

FIG. 4 shows a flow diagram for illustrating a purification process of acarboxylic acid of Comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention shall now be described in detail with reference ifnecessary to the attached drawings.

FIG. 1 is a flow diagram for explaining a production process of acarboxylic acid of the present invention.

This embodiment shows a production process for producing acetic acid(purified acetic acid) from a reaction mixture formed by a continuouscarbonylation reaction of methanol and carbon monoxide in the presenceof a carbonylation catalytic system composed of a rhodium catalyst and aco-catalyst (lithium iodide and methyl iodide).

The process comprises a reactor 3 for carrying out the above-mentionedcarbonylation reaction of methanol; a distillation column (orcatalyst-separating column) 5 for mainly separating the rhodium catalystand lithium iodide from the reaction mixture containing acetic acidgenerated by the reaction; a higher-boiling point (or higher bp)component-separation column (or nonvolatile component-separation column)8 for removing propionic acid; a carboxylic acid-separating column 11for separating a fraction containing at least acetaldehyde from afraction containing acetic acid; and an aldehyde-separating column 14for removing acetaldehyde from the fraction containing at leastacetaldehyde separated by the carboxylic acid-separating column 11. Inthis specification, the term “boiling point” is sometimes referred to as“bp”.

In more detail, the reactor 3 constitutes a liquid-phase reaction systemcontaining a carbonylation catalytic system [a catalytic system composedof a main catalyst component (such as a rhodium catalyst) and aco-catalyst (such as lithium iodide and methyl iodide)]. In such areactor 3, with continuous feeding of methanol as a liquid component ata predetermined rate via a feed line 2, carbon monoxide as a gaseousreaction component is directly and continuously fed via a feed line 1.Since such a liquid-phase reaction system is an exothermic reactionsystem that accompanies generation of heat, the reactor 3 may comprise aheat-removing unit or cooling unit (e.g., jacket) for controlling areaction temperature.

The reaction mixture (or crude reaction solution) formed in the reactor3 comprises, lower-boiling impurities having a boiling point lower thanthat of acetic acid (e.g., acetaldehyde being a precursor of aceticacid) and higher-boiling impurities having a boiling point higher thanthat of acetic acid (e.g., propionic acid) as impurities, in addition tometal catalyst components (a rhodium catalyst, and lithium iodide as aco-catalyst), acetic acid, methyl iodide as a co-catalyst, methylacetate that is a reaction product of acetic acid with methanol, water,or others.

In order to purify acetic acid from such a reaction mixture, withwithdrawing a fraction of the reaction mixture from the reactor 3continuously, the withdrawn reaction mixture is fed to thecatalyst-separating column 5 through a feed line 4. In thecatalyst-separating column 5, a catalyst component having a higher bp(e.g., a metal-containing catalyst component such as the rhodiumcatalyst and lithium iodide) is withdrawn from the column bottom toseparate from the reaction mixture. The higher bp catalyst component (ornonvolatile catalyst component) is a reusable fraction by recycling, andthus after separation with the catalyst-separating column 5 the fractionis recycled to the reaction system (reactor 3) through a first recycleline 7.

The overhead fraction (or lower bp fraction or stream) which isdistilled out from the overhead of the catalyst-separating column 5 andcontains acetic acid is fed to the higher bp component-separation column8 through a feed line 6. In the higher bp component-separation column 8,a bottom fraction (or higher bp fraction or stream) containing at leastpropionic acid is separated from the column bottom through a bottom line10. Propionic acid can be relatively easily separated from acetic acidby utilizing difference between the both in boiling point. In the higherbp component-separation column 8, to adjust the overhead temperature (orthe column bottom temperature), the overhead pressure sets up in a rangeof about 10 to 1,000 kPa as an absolute pressure.

The overhead fraction which is distilled out from the overhead of thehigher bp component-separation column 8 and contains acetic acid is fedto the carboxylic acid-separating column 11 through a feed line 9. Inthe carboxylic acid-separating column 11, an overhead fractioncontaining at least acetaldehyde is separated from the overhead,purified acetic acid can be separated and recovered from the columnbottom through a bottom line 13 as a bottom (or nonvolatile) fraction.

The overhead fraction separated by the carboxylic acid-separating column11 comprises acetaldehyde, and in addition useful components (methyliodide of the co-catalyst, methyl acetate that is a reaction product ofacetic acid with methanol, and water). In order to remove acetaldehydeamong these components and to recycle the useful components to thereaction system, the overhead fraction is further fed to analdehyde-separating column 14 through a feed line 12. Incidentally,acetic acid can be separated from acetaldehyde based on differencebetween the both in boiling point. Thus, in the carboxylicacid-separating column 11 acetic acid can be efficiently separated fromthe overhead fraction containing acetaldehyde. In particular, sincemethyl acetate and methyl iodide act as azeotropic components relativeto water, acetic acid can be highly purified. In the carboxylicacid-separating column 11, to adjust the overhead temperature (or thecolumn bottom temperature), the overhead pressure sets up in a range ofabout 10 to 1,000 kPa as an absolute pressure.

In the aldehyde-separating column 14, the overhead fraction containingacetaldehyde is separated through a distillation line 15 from the columnoverhead, and the bottom fraction containing the useful component (orfraction) is separated from the column bottom.

The bottom fraction separated by the aldehyde-separating column 14usually contains water, methyl iodide being a co-catalyst, methylacetate that is a reaction product of acetic acid with methanol, andothers. In order to utilize these components as a catalyst or reactioncomponent effectively, the bottom fraction is recycled through a secondrecycle line 16 to the reaction system, and is converged in methanol fedfrom the feed line 2 for feeding the reactor 3.

Incidentally, in the aldehyde-separating column 14, to adjust theoverhead temperature, the overhead pressure sets up in a range of about10 to 1,000 kPa as an absolute pressure.

Since such a process insures that methyl acetate and/or methyl iodidemay coexist with water in the carboxylic acid-separating column 11, notonly acetaldehyde but also water that has difficulty separating fromacetic acid may be efficiently removed by azeotrope of the foregoingmethyl iodide or methyl acetate with water. Therefore, acetic acid maybe separated from water without increasing the number of plates of thedistillation column or enhancing the reflux ratio. Moreover, acetic acidcan be efficiently recovered as a finished product without circulating alarge amount of acetic acid within the reaction system. Further, in thealdehyde-separating column, since the vapor pressure of acetaldehyde ishigh in the overhead fraction to be fed, acetaldehyde may be accuratelyseparated from the useful component (or fraction) or the bottomfraction, and deterioration in purification efficiency of acetic acidmay be inhibited by circulating of acetaldehyde within the reactionsystem. Moreover, methyl iodide, water or the like separated by thealdehyde-separating column may be effectively utilized by recycling tothe reaction system. In addition, by recycling water to the reactionsystem, the catalytic system in the reaction system may be stabilized.Therefore, impurities may be separated efficiently at high-energyefficiency, the vapor amount to be used for heating of the higher bpcomponent-separation column to the aldehyde-separating column may bedrastically reduced, and cost of equipment may also be cut down.

FIG. 2 is a flow diagram for illustrating another embodiment of aproduction process of acetic acid of the present invention.

This embodiment shows a process which comprises feeding the overheadfraction separated by the catalyst-separating column in the embodimentof FIG. 1 to a lower bp component-separation column to separate a bottomfraction from an overhead fraction containing at least acetaldehydethrough the lower bp component-separation column, and then feeding thebottom fraction separated by the lower bp component-separation column toa higher bp component-separation column. Such a process is useful as asystem for highly removing an aldehyde from the objective carboxylicacid.

The process comprises a reactor 23 for carrying out the carbonylationreaction of the above-mentioned methanol; a catalyst-separating column25 for mainly separating a higher bp catalyst fraction (or component) (arhodium catalyst and lithium iodide) from the reaction mixturecontaining acetic acid generated by the reaction; a lower bpcomponent-separation column 37 for separating acetaldehyde; a higher bpcomponent-separation column 28 for removing propionic acid; and acarboxylic acid-separating column 31 for separating water. Incidentally,in the same manner as in the embodiment of FIG. 1, carbon monoxide andmethanol may be fed to the reactor through feed lines 21 and 22,respectively.

In this embodiment, similar to the embodiment of FIG. 1, the overheadfraction which is distilled off from the catalyst-separating column 25and contains acetic acid is fed to the lower bp component-separationcolumn 37 through a feed line 26. In the lower bp component-separationcolumn 37, an overhead fraction containing at least acetaldehyde isseparated through a distillation line 38 from the column overhead.Incidentally, since acetaldehyde can be easily separated from aceticacid, in the lower bp component-separation column 37 acetaldehyde can beefficiently discharged or distilled off as an overhead fraction out ofthe system.

In the lower bp component-separation column 37, to adjust the overheadtemperature, the overhead pressure sets up in a range of about 10 to1,000 kPa as an absolute pressure. Incidentally, in the case where theoverhead temperature of the lower bp component-separation column ishigh, not only acetaldehyde but also methyl iodide as a co-catalyst,methyl acetate that is a reaction product of acetic acid with methanol,water, acetic acid, and others are sometimes distilled off as anoverhead fraction. In such a case, acetaldehyde may be further removedfrom the distillate for recycling the residual fraction to the reactionsystem.

The bottom fraction which is withdrawn from the column bottom of thelower bp component-separation column 37 and contains acetic acid is fedto a higher bp component-separation column 28 through a feed line 39. Inthe higher bp component-separation column 28, a bottom fractioncontaining at least propionic acid is separated from the column bottomthrough a bottom line 30. Propionic acid may be relatively easilyseparated from acetic acid using difference between the both in boilingpoint. In the higher bp component-separation column 28, to adjust theoverhead temperature (or the column bottom temperature), the overheadpressure sets up in a range of about 10 to 1,000 kPa as an absolutepressure.

The overhead fraction (liquid or gas) which is distilled off from theoverhead of the higher bp component-separation column 28 and containsacetic acid is further fed to a carboxylic acid-separating column 31through a feed line 29. In the carboxylic acid-separating column 31, theoverhead fraction containing at least water is separated from theoverhead, and purified acetic acid may be separated as a bottom fractionthrough a bottom line 33 from the bottom of the column. In thecarboxylic acid-separating column 31, to adjust the overhead temperature(or the column bottom temperature), the overhead pressure sets up in arange of about 10 to 1,000 kPa as an absolute pressure.

The overhead fraction distilled off from the overhead of the carboxylicacid-separating column 31 comprise water, in addition, methyl iodide asa co-catalyst, methyl acetate that is a reaction product of acetic acidwith methanol, or others. In order to utilize these components as acatalyst or reaction component effectively, the overhead fraction isrecycled to the reaction system through a second recycle line 32, and isconverged in methanol fed from a feed line 22 for feeding to the reactor23. Thus, recycle of water can stabilize the catalytic system in thereaction system.

According to such a process, in the carboxylic acid-separating column31, methyl acetate or methyl iodide coexists with water to allow toazeotrope with water efficiently so that water may be removed.Therefore, acetic acid may be separated from water without increasingthe number of plates of the distillation column or enhancing the refluxratio. Moreover, acetic acid may be recovered as a finished productwithout circulating a large amount of acetic acid within the reactionsystem. As the result, impurities may be separated efficiently at highenergy efficiency, the vapor amount to be used for heating the lower bpcomponent-separation column, the higher bp component-separation columnand the carboxylic acid-separating column may be drastically reduced,and cost of equipment may be also cut down.

FIG. 3 is a flow diagram for illustrating still another embodiment of aproduction process of the present invention.

This embodiment shows a process useful for a system in which an overheadfraction distilled from the lower bp component-separation columncomprises acetaldehyde, in addition methyl iodide as a co-catalyst, andin some cases further comprises methyl acetate, water, and others in theembodiment of FIG. 2.

The process comprises a reactor 43 for carrying out a carbonylationreaction of methanol; a catalyst-separating column 45 for mainlyseparating higher bp catalyst component (a rhodium catalyst and lithiumiodide) from the reaction mixture containing acetic acid formed by thereaction; a lower bp component-separation column 57 for separating atleast acetaldehyde and methyl iodide as a co-catalyst; a higher bpcomponent-separation column 48 for removing propionic acid; a carboxylicacid-separating column 51 for separating at least water; and analdehyde-separating column 54 for removing acetaldehyde from an overheadfraction containing acetaldehyde and methyl iodide which are separatedby the lower bp component-separation column 57. Incidentally, in thesame manner as in the embodiment of FIG. 2, carbon monoxide and methanolmay be fed to the reactor through feed lines 41 and 42, respectively.

In this embodiment, the overhead fraction which is distilled off fromthe overhead of the catalyst-separating column 45 and contains aceticacid is fed to the lower bp component-separation column 57 through afeed line 46 similar to the embodiment of FIG. 2. In the lower bpcomponent-separation column 57, an overhead fraction containing at leastacetaldehyde is separated from the overhead. As described above, in thecase where the distillation temperature (overhead temperature) of thelower bp component-separation column is high, the overhead fraction inthe lower bp component-separation column also comprises methyl iodide,and methyl acetate, water, acetic acid, or others in addition toacetaldehyde. Components such as methyl iodide, methyl acetate, waterand acetic acid may be recycled to the reaction system (reactor 43),however, acetaldehyde deteriorates purification efficiency of aceticacid. Therefore, acetaldehyde is removed by feeding the overheadfraction distilled off from the overhead of the lower bpcomponent-separation column 57 to the aldehyde-separating column 54through a feed line 58.

In the lower bp component-separation column 57, to adjust the overheadtemperature, the overhead pressure sets up in a range of about 10 to1,000 kPa as an absolute pressure.

According to this embodiment, in the lower bp component-separationcolumn 57, since acetaldehyde is separated with a high degree ofaccuracy by enhancing the overhead temperature, the load in the higherbp component-separation column and carboxylic acid-separating column isrelievable, and therefore impurities are removable efficiently.

To the higher bp component-separation column 48 the bottom fraction(i.e., higher bp fraction) withdrawn from the column bottom of the lowerbp component-separation column 57 is fed, and is separated into the twofractions, a bottom fraction which is withdrawn from the bottom andcontains at least propionic acid, and an overhead fraction which isdistilled off from the overhead and contains acetic acid, and theoverhead fraction is fed to the carboxylic acid-separating column 51. Inthe higher bp component-separation column 48, to adjust the overheadtemperature (or the column bottom temperature), the overhead pressuresets up in a range of about 10 to 1,000 kPa as an absolute pressure.

To the carboxylic acid-separating column 51 the overhead fraction (i.e.,lower bp fraction) distilled off from the overhead of the higher bpcomponent-separation column 48 is fed, and is separated into the twofractions, an overhead fraction which is distilled off from the overheadand contains at least water, and a bottom fraction which is withdrawnfrom the column bottom and contains purified acetic acid. The overheadfraction separated from the overhead of the carboxylic acid-separatingcolumn 51 comprises water, methyl iodide as a co-catalyst, methylacetate and others, and is recycled to the reaction system in the samemanner as in the embodiment of FIG. 2. Incidentally, in the carboxylicacid-separating column 51, to adjust the overhead temperature (or thecolumn bottom temperature), the overhead pressure sets up in a range ofabout 10 to 1,000 kPa as an absolute pressure.

The overhead fraction distilled off from the overhead of the lower bpcomponent-separation column 57 is fed to the aldehyde-separating column54 through the feed line 58. In the aldehyde-separating column 54, theoverhead fraction containing acetaldehyde is distilled and removed fromthe overhead of the column through a distillation line 55, and thebottom fraction is separated from the bottom of the column. The bottomfraction separated from the bottom of the column comprises methyliodide, water, methyl acetate, and others. In order to utilize thesecomponents effectively, the bottom fraction is recycled to the reactionsystem (reactor 43) through a third recycle line 56, and is converged inmethanol fed from a feed line 42 for feeding the reactor 43. In thealdehyde-separating column, to adjust the overhead temperature, theoverhead pressure sets up in a range of about 10 to 1,000 kPa as anabsolute pressure.

Such a process ensures accurate separation of acetaldehyde in the lowerbp component-separation column 57, and it can be inhibited thatpurification efficiency of acetic acid deteriorates due to circulationof acetaldehyde through the reaction system. Moreover, sinceacetaldehyde can be separated with a high degree of accuracy in thelower bp component-separation column 57, in the higher bpcomponent-separation column 48 and carboxylic acid-separating column 51the load is relievable, and impurities are separable efficiently.

Further, in the carboxylic acid-separating column 51, in the same manneras in the embodiment of FIG. 2, methyl acetate or methyl iodide coexistswith water to allow to azeotrope with water efficiently so that watermay be removed to be separated from acetic acid. Moreover, acetic acidcan be collected as a finished product without circulating a largeamount of acetic acid through the reaction system.

Moreover, by recycling components such as methyl iodide and waterseparated by the carboxylic acid-separating column 51 and thealdehyde-separating column 54 to the reaction system, these componentsmay be utilized effectively, and in addition, by recycling water to thereaction system, the catalytic system in the reaction system may bestabilized.

Therefore, impurities may be separated efficiently at high energyefficiency, the vapor amount to be used for heating of the lower bpcomponent-separation column to the aldehyde-separating column may bedrastically reduced, and cost of equipment may be also cut down.

The production process of the present invention comprises acarbonylation reaction (i.e., a reaction step) for forming a carboxylicacid and a purification process of the carboxylic acid (including a stepfor separating a higher bp catalyst component, and a step for purifyingthe carboxylic acid), and is applicable for carbonylation reactions ofvarious alcohols or derivatives thereof, not being limited to theforegoing carbonylation reaction of methanol.

(Carbonylation Reaction)

In a carbonylation reaction, an alcohol or a derivative thereof(reactive derivative) is carbonylated with carbon monoxide. As thealcohol to be used in the carbonylation reaction, there may beexemplified an alcohol having “n” carbon atom(s), for example, analiphatic alcohol [e.g., an alkanol (e.g., a C₁₋₁₀alkanol) such asmethanol, ethanol, propanol, isopropanol, butanol, pentanol, orhexanol], an alicyclic alcohol [e.g., a cycloalkanol (e.g., a C₃₋₁₀cycloalkanol) such as cyclohexanol or cyclooctanol], an aromatic alcohol[an aryl alcohol (e.g., a C₆₋₁₀aryl alcohol (such as a phenol compound))such as phenol; an aralkyl alcohol (e.g., a C₆₋₁₀aryl-C₁₋₄alkanol) suchas benzyl alcohol or phenethyl alcohol], or others. The number “n” ofcarbon atom is about 1 to 14, preferably about 1 to 10, and morepreferably about 1 to 6. Among the foregoing alcohols, an aliphaticalcohol is preferred. The number “n” of carbon atom in the aliphaticalcohol is, for example, about 1 to 6, preferably about 1 to 4, and inparticular about 1 to 3.

Among the alcohol derivatives, an ester compound includes an ester of acarboxylic acid to be formed with a raw alcohol, for example, aC₁₋₆alkyl ester of a C₂₋₆carboxylic acid such as methyl acetate or ethylpropionate, or others. An ether compound includes an ether correspondingto the raw alcohol, for example, a diC₁₋₆alkyl ether such as methylether, ethyl ether, propyl ether, isopropyl ether or butyl ether, or thelike. If necessary, as an alcohol, there may be used a polyhydricalcohol, for example, an alkylene glycol such as ethylene glycol,propylene glycol or butanediol, or a derivative thereof (e.g., an ester,a halide, an ether).

The alcohol or a derivative thereof may be used singly or incombination.

In the preferred liquid-phase reaction system, an alcohol having “n”carbon atom(s) as a liquid reaction component, preferably a C₁₋₄alcoholor a derivative thereof (e.g., methanol, methyl acetate, methyl iodide,dimethyl ether) may be used to obtain a carboxylic acid having “n+1”carbon atoms or a derivative thereof (e.g., a carboxylic anhydride). Inparticular, the following reaction system is preferred: a reactionsystem in which at least one member selected from the group consistingof methanol, methyl acetate, and dimethyl ether (particularly at leastmethanol) is allowed to react with carbon monoxide in the presence of acarbonylation catalyst or a catalytic system in a liquid-phase reactionsystem, to produce acetic acid or a derivative thereof.

Incidentally, the alcohol or a derivative thereof may be directly fed tothe reaction system without going through the recycle line. Moreover, analcohol or a derivative thereof distilled off from a purification step(for example, a carboxylic acid-separating column shown in FIG. 2, or analdehyde-separating column shown in FIGS. 1 and 3) may be usually fed toa reactor through a recycle line.

The liquid-phase reaction can be carried out in the presence of variouscatalytic systems, not being limited to the foregoing catalytic system.The catalytic system usually comprises a carbonylation catalyst, and aco-catalyst or accelerator.

As the carbonylation catalyst, there may be usually employed a catalysthaving a high boiling point, e.g., a metal catalyst. Such a catalystincludes a transition metal catalyst, in particular a metal catalystcontaining a metal element of the Group 8 of the Periodic Table ofElements, for example, a cobalt catalyst, a rhodium catalyst, an iridiumcatalyst, or others. The catalyst may be a simple metal, or may be usedin the form of a metal oxide (including a complex metal oxide), an metalhydroxide, a metal halide (e.g., a chloride, a bromide, a iodide), ametal carboxylate (e.g., an acetate), a metal salt of an inorganic acid(e.g., a sulfate, a nitrate, a phosphate), a metal complex or others.Such a metal catalyst may be used singly or in combination.

The preferred metal catalyst includes a rhodium catalyst and an iridiumcatalyst (in particular a rhodium catalyst). Incidentally, rhodiumusually exists as a complex in a reaction solution, and in the caseusing a rhodium catalyst, the catalyst is not particularly limited asfar as the catalyst can change into a complex in a reaction solution,and may be used in various forms. As such a rhodium catalyst, a rhodiumhalide (such as bromide or iodide) is particularly preferred. Moreover,the catalyst may be stabilized in a reaction solution by adding a saltof a halide (e.g., a salt of an iodide) and/or water thereto.

The concentration of the catalyst is, for example, about 5 to 10,000ppm, preferably about 10 to 7,000 ppm, more preferably about 20 to 5,000ppm (e.g., about 50 to 5,000 ppm), and in particular about 100 to 2,000ppm on the basis of weight relative to the total amount of theliquid-phase system.

As the co-catalyst or accelerator constituting the catalytic system,there may be used, not being limited to the foregoing lithium iodide andmethyl iodide, various alkali metal halides (e.g., a iodide such aspotassium iodide or sodium iodide; a bromide such as lithium bromide,potassium bromide or sodium bromide), a hydrogen halide (e.g., hydrogeniodide, hydrogen bromide), an alkyl halide [an alkyl halide (aC₁₋₁₀alkyl halide, preferably a C₁₋₄alkyl halide) corresponding to a rawalcohol, for example a C₁₋₁₀alkyl iodide (e.g., a C₁₋₄alkyl iodide) suchas methyl iodide, ethyl iodide or propyl iodide, a bromide correspondingto the alkyl iodide (e.g., methyl bromide, propyl bromide), or achloride corresponding to the alkyl iodide (e.g., methyl chloride)].Incidentally, the alkali metal halide (in particular a salt of aniodide) also functions as a stabilizer of a carbonylation catalyst(e.g., a rhodium catalyst). The co-catalyst(s) or accelerator(s) may beused singly or in combination. In particular, an alkali metal halide (inparticular an alkali metal iodide) and an alkyl halide (in particular analkyl iodide) are preferably used in combination.

The content of the co-catalyst or accelerator is about 0.1 to 40% byweight, preferably about 0.5 to 30% by weight, and more preferably about1 to 25% by weight relative to total amount of the liquid-phase system.More specifically, in a production of a carboxylic acid by the foregoingcarbonylation reaction of an alcohol, the content of the alkyl halidesuch as methyl iodide is about 0.1 to 30% by weight, preferably about 1to 25% by weight, and more preferably about 5 to 20% by weight relativeto the total amount of the liquid-phase system, and the content of thealkali metal halide such as lithium iodide is about 0.1 to 50% byweight, preferably about 0.5 to 40% by weight, and more preferably about1 to 30% by weight relative to the total amount of the liquid-phasesystem.

Incidentally, in the reaction system a carboxylic acid ester (inparticular an ester of a carboxylic acid with an alcohol, such as methylacetate) may be included in a proportion of about 0.1 to 75% by weight,preferably about 0.2 to 50% by weight (e.g., about 0.2 to 25% byweight), and more preferably about 0.5 to 20% by weight (e.g., about 1to 10% by weight) relative to the total amount of the liquid-phasesystem.

Carbon monoxide may be used as a pure gas, or may be used as a gasdiluted with an inert gas (e.g., nitrogen, helium, carbon monoxide). Thepartial pressure of carbon monoxide in the reaction system may beselected suitably depending on the species of reaction, and others. Forexample, in a production of a carboxylic acid by a carbonylationreaction of an alcohol, the partial pressure of carbon monoxide in thereaction system is, for example, about 200 to 3,000 kPa, preferablyabout 400 to 2,000 kPa, and more preferably about 500 to 2,000 kPa as anabsolute pressure.

Incidentally, carbon monoxide may be fed from the lower part of thereactor by sparging.

The reaction may be carried out in the presence or absence of a solvent,or carried out in the presence of a hydrogen gas.

Moreover, the reaction may be conducted in the presence of water. Theexistence of water in the reaction system is important because wateracts on stability of a metal catalyst (such as a rhodium catalyst) andgenerating rate of an objective carboxylic acid (such as acetic acid).However, in the case where the proportion of water is too much in thereaction system, it is difficult to separate water efficiently in thepurification step. Therefore, the proportion of water in the reactionsystem is usually not more than 20% by weight (e.g., about 0.001 to 20%by weight), preferably about 0.01 to 20% by weight, and more preferablyabout 0.1 to 15% by weight (e.g., about 1 to 15% by weight) as the watercontent in the liquid-phase system (or the reaction solution). In thecase where the water content is too small, stability of the metalcatalyst (such as a rhodium catalyst) and generating rate of thecarboxylic acid (such as acetic acid) are deteriorated, and there is apossibility of significantly generating by-product (such as aceticanhydride). When the water content is over 20% by weight, the amount ofwater to be separated and recycled increases in the purification step,the vapor amount to be heated or the separation and removal equipmentgrow large, then too much costs are needed, as a result this isunfavorable industrially.

In the carbonylation reaction, the reaction temperature may for examplebe about 100 to 250° C. (preferably about 150 to 220° C., morepreferably about 170 to 210° C.), and the reaction pressure (absolutepressure) may for example be about 1,000 to 5,000 kPa (e.g., about 1,500to 4,000 kPa).

In the foregoing carbonylation reaction, a carboxylic acid having “n+1”carbon atoms (e.g., acetic acid) corresponding to an alcohol having “n”carbon atom(s) (e.g., methanol) is formed together with an ester of theformed carboxylic acid with the alcohol (e.g., methyl acetate), watergenerated with the esterification reaction, in addition an aldehydehaving “n+1” carbon atoms (e.g., acetaldehyde) corresponding to thealcohol, a carboxylic acid having “n+2” carbon atoms (e.g., propionicacid), and others.

(Purification of Carboxylic Acid)

In the present invention, a carboxylic acid is purified from acarbonylation product by a separation step (A) of a higher bp catalystcomponent (or metal catalytic component), and a purification step (B) ofa carboxylic acid.

(A) Separation Step of Higher Bp Catalyst Component

In a separation step of the higher bp catalyst component, the higher bpcatalyst component (metal catalytic component, e.g., a carbonylationcatalyst such as a rhodium catalyst, and an alkali metal halide) isseparated from the reaction mixture obtained from the above-mentionedreaction system. The separation of the higher bp catalyst component maybe conducted by a conventional separation method or separationapparatus, and may be usually carried out with the use of a distillationcolumn (e.g., a plate column, a packed column, a flash distillationcolumn). Moreover, the metal catalytic component may be separated bymeans of distillation in combination with a mist- or solid-collectingmethod which is widely used in industrial application.

The reaction mixture is separated into a vapor component as an overheadfraction containing the reaction product and a liquid component as abottom fraction by distillation. In the separation step, the reactionmixture may be heated, or may be separated into the vapor component andthe liquid component without heating. For example, when flashdistillation is utilized, in adiabatic flash the reaction mixture may beseparated into the vapor component and the liquid component with notheating but reduced pressure, and in thermostatic flash the reactionmixture may be separated into the vapor component and the liquidcomponent with heating and reduced pressure. The reaction mixture may beseparated into the vapor component and the liquid component by combiningthese flash conditions. The flash distillation step may be carried outat a temperature of about 80 to 200° C. under a pressure (absolutepressure) of about 50 to 1,000 kPa (e.g., about 100 to 1,000 kPa).

The separation step of the catalyst may be composed of a single step, ormay be composed of a plurality of steps in combination. The bottomfraction (or metal catalytic fraction) separated by such a step isusually recycled to the reaction system.

Incidentally, the bottom fraction containing a higher bp catalystcomponent and a carboxylic acid having “n+2” carbon atoms may beseparated from the reaction mixture to separate the bottom fraction intothe higher bp catalyst component and the carboxylic acid having “n+2”carbon atoms.

(B) Purification Step

In the purification step, a carboxylic acid may be purified with the useof the higher bp component-separation column and the carboxylicacid-separating column. Moreover, a crude mixture containing an aldehydemay be fed to the higher bp component-separation column, or a crudemixture in which an aldehyde is separated (or removed) by the lower bpcomponent-separation column beforehand may be fed to the higher bpcomponent-separation column. The fraction (or overhead fraction)containing an aldehyde separated in a purification process of acarboxylic acid usually comprises useful components [e.g., an ester of acarboxylic acid having “n+1” carbon atoms with an alcohol having “n”carbon atom(s), an alkyl halide, water], and may be separated into theuseful components and an aldehyde which deteriorates quality of anobjective carboxylic acid by means of the following separation unit(aldehyde-separating column) for recycling the useful components to thereaction system.

In the purification step, for example, a carboxylic acid may beefficiently purified with the following systems: (b1) a systemcomprising a higher bp component-separation column, a carboxylicacid-separating column, and an aldehyde-separating column in that order;(b2) a system comprising a lower bp component-separation column, ahigher bp component-separation column, and a carboxylic acid-separatingcolumn in that order; and (b3) a system comprising a lower bpcomponent-separation column, a higher bp component-separation column, acarboxylic acid-separating column, and an aldehyde-separating column inthat order. Incidentally, as the lower bp component-separation column,the higher bp component-separation column, the carboxylicacid-separating column, and the aldehyde-separating column, for example,there may be used conventional distillation columns such as a platecolumn, a packed column, and a flash distillation column.

The overhead fraction (crude mixture) in which the higher bp catalystcomponent is removed by the catalyst-separating column is usuallycomposed mainly of an aldehyde having “n+1” carbon atoms, a carboxylicacid having “n+2” carbon atoms, a carboxylic acid having “n+1” carbonatoms, an ester of the carboxylic acid having “n+1” carbon atoms with analcohol having “n” carbon atom(s), an alkyl halide, water, or others. Ifnecessary, the aldehyde contained in the overhead fraction (crudemixture) may be fed to the lower bp component-separation column forseparating from the crude mixture as an overhead fraction beforehand, ormay be separated from the crude mixture in an appropriate step (analdehyde-separating column).

(1) Lower Bp Component-Separation Column

The distillation temperature (or overhead temperature) and pressure (oroverhead pressure) in the lower bp component-separation column may beselected depending on the species of aldehyde, an objective carboxylicacid and distillation column, and is not particularly limited as far asat least an aldehyde having “n+1” carbon atoms (preferably the aldehyde,and an alkyl halide such as methyl iodide) is separable by usingdifference between of an aldehyde to be separated as an overheadfraction and a bottom fraction in boiling point. For example, in thecase carrying out purification of acetic acid by a plate column, theoverhead pressure is about 10 to 1,000 kPa, preferably about 10 to 700kPa, and more preferably about 50 to 500 kPa as an absolute pressure. Inthe case where the overhead pressure is too low, the boiling point of analdehyde (in particular acetaldehyde in purification of acetic acid)becomes low, it is necessary to lower the temperature for condensinggaseous components, and as a result it is not preferred in cost. On theother hand, in the case where the overhead pressure is too high, theinner temperature of the column rises due to excessively added pressure,as a result there is a possibility that an aldehyde (in particularacetaldehyde) condensed within the column is polymerized within thecolumn by exposing to high temperature.

Moreover, the overhead temperature may be adjusted by adjusting theoverhead pressure, and may for example be about 20 to 180° C.,preferably about 30 to 150° C., and more preferably about 40 to 120° C.As described above, in the lower bp component-separation column, analkyl halide, a carboxylic acid ester, water or other components inaddition to the aldehyde may be also separated as an overhead fractionby enhancing the overhead temperature. Such an overhead fraction may befed to an aldehyde-separating column, and may be separated into analdehyde and useful components (an alkyl halide, a carboxylic acidester, and water). In such a case, the overhead temperature may be about20 to 180° C., preferably about 30 to 150° C., and more preferably about40 to 120° C.

Moreover, in the case of a plate column, the theoretical plate number isnot particularly limited, and is about 5 to 30, preferably about 7 to25, and more preferably about 8 to 20 depending on the species ofcomponent (or fraction) to be separated. Further, in order to highly (oraccurately) separating an aldehyde by the fraction column, thetheoretical plate number may be about 20 to 80, preferably about 25 to60, and more preferably about 30 to 50. The removal of an aldehyde witha distillation column having such a plate number ensures significantdecrease of load in a following distillation column.

In the lower bp component-separation column, the reflux ratio may forexample be selected from about 0.5 to 3,000, and preferably about 1 to2,000 depending on the above-mentioned theoretical plate number. As thetheoretical plate number becomes larger, usually the reflux ratio may besmaller. Incidentally, the overhead fraction obtained by removing thebottom fraction in the separation step of the catalyst component is notnecessarily subjected to reflux, and may be fed from the overhead of thelower bp component-separation column.

The bottom fraction separated by the lower bp component-separationcolumn is usually composed mainly of a carboxylic acid having “n+2”carbon atoms, a carboxylic acid having “n+1” carbon atoms, an ester, analkyl halide, water, and others.

(2) Higher Bp Component-Separation Column

In the higher bp component-separation column, noticing the viewpointthat a carboxylic acid having “n+1” carbon atoms and a carboxylic acidhaving “n+2” carbon atoms can be efficiently separated by utilizingdifference between the both in boiling point, a carboxylic acid having“n+2” carbon atoms (e.g., propionic acid) is removed out of the systemas a bottom fraction from the overhead fraction separated by thecatalyst-separating column or the bottom fraction separated by the lowerbp component-separation column. Therefore, propionic acid may be easilyand accurately separated from acetic acid.

The distillation temperature and pressure in the higher bpcomponent-separation column is not particularly limited as far as atleast a carboxylic acid having “n+2” carbon atoms (e.g., propionic acid)is separable as a bottom fraction from an objective carboxylic acid (acarboxylic acid having “n+1” carbon atoms) by using difference betweenof the both in boiling point, and may be selected depending on thespecies of the foregoing carboxylic acid having “n+1” carbon atoms andcarboxylic acid having “n+2” carbon atoms as well as that ofdistillation column.

For example, in the case purifying acetic acid as an objectivecarboxylic acid by a plate column, the overhead pressure is about 10 to1,000 kPa, preferably about 10 to 700 kPa, and more preferably about 50to 500 kPa as an absolute pressure. In the case where the overheadpressure is too low, the separation efficiency of the overhead fractionsuch as acetic acid, water, methyl iodide, and in some casesacetaldehyde becomes down, it is necessary to lower the temperature forcondensing gaseous components efficiently, and as a result there is apossibility of cost thereof bringing disadvantages. On the other hand,in the case where the overhead pressure is too high, an excessivepressure is added to the column thereby increasing the bottomtemperature, and further based on this, the pressure of vapor to beheated rises. As a result, equipment back up is required, and there is apossibility of cost thereof bringing disadvantages.

Moreover, the temperature of the column bottom may be adjusted byadjusting the overhead pressure. For example, in the case utilizing aplate column for purification of acetic acid, the temperature of thecolumn bottom is not higher than 170° C. (e.g., about 50 to 170° C.),preferably about 70 to 170° C., more preferably about 100 to 170° C.Incidentally, in the case separating a carboxylic acid having “n+2”carbon atoms from the bottom fraction in which an aldehyde is separatedby the lower bp component-separation column beforehand, the temperatureof the column bottom of the higher bp component-separation column mayfor example be about 130 to 170° C., preferably about 140 to 170° C.,and more preferably about 150 to 170° C. In the higher bpcomponent-separation column, when the temperature of the column bottomis over 170° C., there is a possibility that acetic anhydride isgenerated by dehydration of acetic acid in the column bottom, and thatthe resultant acetic anhydride is distilled out from the overhead of thecolumn to contaminate in acetic acid as a finished product.

In the case of the plate column, the theoretical plate number is notparticularly limited, and is about 5 to 30, preferably about 7 to 25,and more preferably about 8 to 20 depending on the species of componentto be separated.

Moreover, in the case of separating an aldehyde by the lower bpcomponent-separation column beforehand, the theoretical plate number ofthe higher bp component-separation column may be about 7 to 30,preferably about 8 to 25, and more preferably about 10 to 20, and may beusually more than the theoretical plate number of the lower bpcomponent-separation column. Incidentally, as described above, in thecase where an aldehyde is highly separated beforehand with the use ofthe lower bp component-separation column having a large number of thetheoretical plate, other lower-boiling impurities along with an aldehydeare also separated by the lower bp component-separation column, and as aresult in the higher bp component-separation column, the overheadfraction and the bottom fraction can be accurately separated with theuse of a distillation column whose theoretical plate number is less thanthat of the lower bp component-separation column. In such a case, thetheoretical plate number of the higher bp component-separation columnmay for example be about 15 to 60, preferably about 15 to 50, and morepreferably about 20 to 40.

In the higher bp component-separation column, the reflux ratio may forexample be selected from about 0.5 to 10, and preferably about 0.7 to 5depending on the theoretical plate number described above. The refluxratio may be usually reduced by increasing the number of the theoreticalplate. Incidentally, the overhead fraction obtained by removing thebottom fraction in the separation step of the catalytic fraction is notnecessarily subjected to reflux, and may be fed from the overhead of thelower bp component-separation column.

Moreover, in the case separating an aldehyde by the lower bpcomponent-separation column beforehand, the reflux ratio of the higherbp component-separation column may for example be about 0.1 to 10, andpreferably about 0.5 to 5 (e.g., about 0.7 to 5) depending on thetheoretical plate number described above.

The overhead fraction separated by the higher bp component-separationcolumn is usually composed mainly of an aldehyde having “n+1” carbonatoms, a carboxylic acid having “n+1” carbon atoms, an ester, an alkylhalide, water, and others. Incidentally, in the case separating analdehyde by the lower bp component-separation column beforehand, theoverhead fraction mainly comprises the carboxylic acid having “n+1”carbon atoms, the ester, the alkyl halide, water, and others except thealdehyde.

Incidentally, when a salt of an iodide (e.g., a alkali metal iodide, analkyl iodide) is use as a co-catalyst, hydrogen iodide is generated asreduction product by the action of water. Since the resultant hydrogeniodide produces azeotropic mixture having a maximum boiling point (127°C.) by an action of water thereby failing to separate from hydrouscarboxylic acid (e.g., acetic acid), there is a possibility thathydrogen iodide is contaminated into acetic acid as a finished product.Therefore, in the higher bp component-separation column, along withadjusting the heating conditions (e.g., temperature, pressure), acondensed part of hydrogen iodide is formed in the column, a fractionwhich is eluted from the condensed part of hydrogen iodide by side cutand contains hydrogen iodide may be recycled to the reaction system; ora substrate alcohol (e.g., methanol) may be fed to the condensed part(or preferably a fraction containing a side cut hydrogen iodide) toconvert hydrogen iodide to an alkyl iodide (e.g., methyl iodide), andthen recycled to the reaction system. By such a method, higher qualityacetic acid may be obtained.

(3) Carboxylic Acid-Separating Column

The overhead fraction which is separated by the higher bpcomponent-separation column and contains a carboxylic acid having “n+1”carbon atoms usually comprises water (e.g., water generated byesterification), an alkyl halide, a carboxylic acid ester, and in somecases an aldehyde having “n+1” carbon atoms. Therefore, the alkyl halideand/or the carboxylic acid ester may be utilized as an azeotropiccomponent of water, and water may be separated from the fractioncontaining a carboxylic acid having “n+1” carbon atoms efficiently bydistilling in the presence of the ester and/or the alkyl halide as wellas water.

The distillation temperature (or overhead temperature or bottomtemperature) and pressure (or overhead pressure) in the carboxylicacid-separating column is not particularly limited as far as componentssuch as water, an carboxylic acid ester, an alkyl halide, and in somecases an aldehyde are separable as an overhead fraction (or azeotropiccomponent) from an objective carboxylic acid as a bottom fraction byutilizing difference between of the overhead fraction and the objectivecarboxylic acid in boiling point. The temperature and pressure may beselected depending on the species of overhead fraction and the objectivecarboxylic acid as well as distillation column. For example, in the casecarrying out purification of acetic acid by a plate column, the overheadpressure may be about 10 to 1,000 kPa, preferably about 10 to 700 kPa,and more preferably about 50 to 500 kPa as an absolute pressure. In thecase where the overhead pressure is too low, separation efficiency ofthe overhead fraction [water, methyl iodide, methyl acetate, and in somecases an aldehyde (particularly acetaldehyde in purification of aceticacid)] becomes low, it is necessary to lower the temperature forcondensing gaseous components efficiently, and as a result it is notpreferred in cost. On the other hand, in the case where the overheadpressure is too high, the inner temperature of the column rises due toexcessively added pressure, and there is a possibility that an aldehyde(in particular acetaldehyde) which is condensed within the column ispolymerized within the column by exposing to high temperature when analdehyde is present into the column. Further, since the pressure ofvapor to be heated rises, equipment back up is required, and there is apossibility of cost thereof bringing disadvantages.

The temperature of the column bottom may be adjusted by adjusting theoverhead pressure. For example, in the case utilizing a plate column forpurification of acetic acid, the temperature of the column bottom is notmore than 170° C. (e.g., about 50 to 170° C.), preferably about 70 to170° C., more preferably about 90 to 170° C. Moreover, in the case wherean aldehyde is separated by the lower bp component-separation columnbeforehand, the bottom temperature of the carboxylic acid-separatingcolumn may for example be about 130 to 170° C., preferably about 140 to170° C., and more preferably about 150 to 170° C. When the temperatureof the column bottom of the carboxylic acid-separating column is over170° C., there is a possibility that acetic anhydride is formed in thecolumn bottom by dehydration of acetic acid, and that the resultantacetic anhydride is contaminated in acetic acid as a finished product.

In the case of a plate column, the theoretical plate number is notparticularly limited, and is about 20 to 60, preferably about 25 to 55,and more preferably about 30 to 50 depending on the species of component(or fraction) to be separated, and may be usually more than thetheoretical plate number of the higher bp component-separation column.

Moreover, in the case where an aldehyde is separated by the lower bpcomponent-separation column beforehand, the theoretical plate number ofthe carboxylic acid-separating column is not particularly limited, andis about 10 to 80, preferably about 15 to 60 (e.g., about 15 to 50), andmore preferably about 20 to 50 (e.g., about 30 to 50) depending on thespecies of component (or fraction) to be separated, and may be usuallymore than the theoretical plate number of the higher bpcomponent-separation column. Moreover, in the case where an aldehyde ishighly separated with the use of the lower bp component-separationcolumn having a large number of the theoretical plate, otherlower-boiling impurities along with an aldehyde are also separated bythe lower bp component-separation column, as well as impurities are alsoseparated by the higher bp component-separation column efficiently.Therefore, in the carboxylic acid-separating column, the overheadfraction and the bottom fraction may be accurately separated with theuse of a distillation column whose theoretical plate number is less thanthat of the lower bp component-separation column and/or the higher bpcomponent-separation column. In such a case, the theoretical platenumber of the carboxylic acid-separating column may be about 7 to 50,preferably about 8 to 40, and more preferably about 10 to 30.

In the carboxylic acid-separating column, the reflux ratio may forexample be selected from about 0.5 to 20, and preferably about 1 to 10depending on the above-mentioned theoretical plate number. Moreover, inthe case where an aldehyde is separated with the use of the lower bpcomponent-separation column beforehand, the reflux ratio of thecarboxylic acid-separating column may for example be about 1 to 100, andpreferably about 1.5 to 80 depending on the theoretical plate number.

The overhead fraction separated from the carboxylic acid-separatingcolumn usually comprises a mainly aldehyde having “n+1” carbon atoms, inaddition azeotropic components or useful components such as an ester, analkyl halide, and water. The useful components may be separated from thealdehyde by a following separating unit (an aldehyde-separating column)to recycle to the reaction system. Moreover, in the case where thealdehyde in the lower bp component-separation column is separatedbeforehand, the above-mentioned overhead fraction mainly comprisesuseful components such as an ester, an alkyl halide and water, and maybe recycled to the reaction system.

Incidentally, when hydrogen iodide is present in the carboxylicacid-separating column, in the carboxylic acid-separating column, alongwith adjusting the heating conditions (e.g., temperature, pressure), acondensed part of hydrogen iodide is formed in the column, a fractionwhich is eluted from the condensed part of hydrogen iodide by side cutand contains hydrogen iodide may be recycled to the reaction system; ora substrate alcohol (e.g., methanol) may be fed to the condensed part(or preferably a fraction containing a side cut hydrogen iodide) toconvert hydrogen iodide into an alkyl iodide (e.g., methyl iodide), andthen recycled to the reaction system.

Moreover, the objective carboxylic acid may be improved in purity byconverting hydrogen iodide present in the carboxylic acid-separatingcolumn into an alkyl iodide (e.g., methyl iodide) or the like throughfeeding or infusing a substrate alcohol (e.g., methanol) or others inthe column, and by separating the objective carboxylic acid from theconverted product as a overhead fraction. The overhead fraction furthercontaining useful components such as water may be recycled to thereactor.

Moreover, in order to improve a carboxylic acid (e.g., acetic acid) as afinished product in purity, the carboxylic acid as a finished productmay be taken out from a site close to the column bottom of thecarboxylic acid-separating column by side cut, or may be inhibited fromcontamination of a reducing substance (e.g., an aldehyde such asacetaldehyde or crotonaldehyde) by subjecting the reducing substance toozone treatment. Further, after distillation off of the carboxylic acidas a finished product, impurities (e.g., an alkyl iodide such as hexyliodide) may be removed by treating with an ion exchange resin exchangedwith silver to improve the purity of the carboxylic acid.

Such a process insures production of higher quality acetic acid.

In the distillation step for separating the higher bp catalystcomponent, the lower bp component-separation column, the higher bpcomponent-separation column, and the carboxylic acid-separating column,the overhead fraction may be fed to the subsequent step or subsequentseparation column (or distillation column) in the form of gas, and maybe usually fed to the subsequent step or subsequent separation column(or distillation column) in the form of liquid by condensation.

(4) Aldehyde-Separating Column

In the case feeding the overhead fraction which is separated by thecatalyst-separating column and contains an aldehyde to the higher bpcomponent-separation column, the overhead fraction separated from thecarboxylic acid-separating column usually comprises, in addition to analdehyde, water, an alkyl halide (methyl iodide), a carboxylic acidester (methyl acetate), an objective carboxylic acid, and others.Therefore, the overhead fraction separated from the carboxylicacid-separating column may further be fed to an aldehyde-separatingcolumn to remove an aldehyde as an overhead fraction, and the resultantbottom fraction (containing water, an alkyl halide, a carboxylic acidester, an objective carboxylic acid) may be recycled to the reactionsystem. Incidentally, since the aldehyde (e.g., acetaldehyde) has highervapor pressure compared with other impurities, it is possible toseparate the aldehyde easily by the aldehyde-separating column.

Moreover, when an aldehyde is highly separated in the lower bpcomponent-separation column beforehand, the overhead fraction separatedfrom the lower bp component-separation column usually comprises, inaddition to an aldehyde (e.g., acetaldehyde), an alkyl halide (e.g.,methyl iodide), water, an carboxylic acid ester (e.g., methyl acetate),and others. In such a case, the aldehyde may be further fed to thealdehyde-separating column to be removed as an overhead fraction, andthe resultant bottom fraction (containing an alkyl halide, water, acarboxylic acid ester, an objective carboxylic acid) may be recycled tothe reaction system.

The temperature (overhead temperature) and pressure (overhead pressure)in the aldehyde-separating column may be selected depending on thespecies of aldehyde and alkyl halide as well as distillation column, andis not particularly limited as far as at least an aldehyde (e.g.,acetaldehyde) is separable as an overhead fraction from the overheadfraction obtained in the higher bp component-separation column byutilizing difference between the aldehyde and other components(particularly an alkyl halide) in boiling point. For example, in thecase using a plate column as the aldehyde-separating column forpurification of acetic acid, the overhead pressure is about 10 to 1,000kPa, preferably about 10 to 700 kPa, and more preferably about 10 to 500kPa as an absolute pressure. In the case where the overhead pressure istoo low, separation efficiency of acetaldehyde becomes low, it isnecessary to lower the temperature for condensing gaseous componentsefficiently, and as a result it is not preferred in cost. On the otherhand, in the case where the overhead pressure is too high, the innertemperature of the column rises due to excessively added pressure, as aresult there is a possibility that acetaldehyde which is condensedwithin the column is polymerized within the column by exposing to hightemperature thereby being contaminated in the bottom fraction.

Moreover, the overhead temperature may be adjusted by adjusting theoverhead pressure, and for example, is about 10 to 80° C., preferablyabout 20 to 70° C., and more preferably about 40 to 60° C.

In the case where the aldehyde-separating column is a plate column, thetheoretical plate number may be usually more than the theoretical platenumber of the higher bp component-separation column, or may for examplebe about 5 to 40, preferably about 8 to 35, and more preferably about 10to 30 depending on the species of component (or fraction) to beseparated. Moreover, in the case where the overhead fraction which isseparated by the lower bp component-separation column and contains analdehyde is fed to the aldehyde-separating column, the theoretical platenumber of the aldehyde-separating column may be, in a plate column,usually more than the theoretical plate number of the lower bpcomponent-separation column, and may for example be selected from about10 to 80, preferably about 20 to 60, and more preferably about 30 to 50depending on the species of component (or fraction) to be separated.

In the aldehyde-separating column, the reflux ratio may be selected fromabout 1 to 1,000, preferably about 10 to 800, and preferably about 50 to600 (e.g., about 100 to 600) depending on the above-mentionedtheoretical plate number.

Incidentally, when the overhead fraction (or crude mixture) which isseparated by the catalyst-separating column and contains an aldehyde isfed to the higher bp component-separation column, hydrogen iodide existsin the aldehyde-separating column in some cases. In such a case,hydrogen iodide present in the aldehyde-separating column may beconverted into an alkyl iodide (e.g., methyl iodide) through feeding orinfusing a substrate alcohol (e.g., methanol) or others in thealdehyde-separating column to separate as a bottom fraction in thecolumn for recycling to the reaction system.

In the present invention, such a separation and purification processinsures larger energy efficiency, and significant reduction of a vaporamount to be used per 1,000 g of a carboxylic acid compared with aconventional purification process. For example, the vapor amount to berequired for heating in purification of acetic acid [e.g., (1) a higherbp component-separation column, a carboxylic acid-separating column, andan aldehyde-separating column, (2) a lower bp component-separationcolumn, a higher bp component-separation column, and a carboxylicacid-separating column, (3) a lower bp component-separation column, ahigher bp component-separation column, a carboxylic acid-separatingcolumn, and an aldehyde-separating column] is about 500 to 2,000 g,preferably about 500 to 1,500 g, and more preferably about 600 to 1,000g relative to 1,000 g of acetic acid.

According to the present invention, at least a carboxylic acid having“n+2” carbon atoms is removed from a reaction mixture formed by acarbonylation reaction, and then the distillation can be carried out inthe presence of at least water and an ester of a carboxylic acid with analcohol, wherein water and ester are generated in the reaction system.Accordingly, impurities are efficiently separated from the reactionmixture to produce a carboxylic acid (in particular acetic acid) easilyand efficiently. Moreover, a purified carboxylic acid may be producedwith removing water without circulating an excess amount of a carboxylicacid (in particular acetic acid) through the system. Further, since anester and water formed in the reaction system can be utilized asazeotropic components, a carboxylic acid (in particular acetic acid) canbe highly purified without adding an azeotropic component, andtherefore, a highly purified carboxylic acid (in particular acetic acid)can be produced at high energy efficiency.

INDUSTRIAL APPLICABILITY

According to the present invention, in a series of steps as describedabove, particularly in a carboxylic acid-separating column, since acarboxylic acid ester or an alkyl halide (such as methyl iodide) capableof azeotrope with water may coexist with water, water may be removedefficiently without circulating an excess amount of a carboxylic acidthrough the reaction system. Moreover, an aldehyde may be efficientlyremoved with the use of a lower bp component-separation column oraldehyde-separating column. Therefore, a carboxylic acid (e.g., aceticacid) may be highly purified at high-energy efficiency and a low cost,and both of energy cost and equipment expenses may be reduced.Accordingly, the present invention is useful for industrial productionof a carboxylic acid.

EXAMPLES

The following examples are intended to describe this invention infurther detail and should by no means be interpreted as defining thescope of the invention. Incidentally, in Examples, pressure is shown inabsolute pressure.

Example 1

(1) Carbonylation Reaction

A rhodium catalyst, lithium iodide, methyl iodide, and water weresupplied to a reactor at prescribed amounts so that the concentration ofthe rhodium catalyst, that of lithium iodide, that of methyl iodide, andthat of water were 400 ppm, 0.5 mol/L, 14% by weight, and 8% by weightin a mixture (liquid-phase system), respectively. The reaction wascarried out at 187° C. with feeding carbon monoxide and methanol to thereactor continuously to form acetic acid.

(2) Separation Step of Higher Bp Catalyst Component

The reaction mixture (or crude reaction solution) obtained in thereaction step (1) was distilled with the use of a distillation column(catalyst-separating column) (temperature of 132° C., pressure of 252kPa), and was separated into a less-volatile phase (bottom fraction) anda higher-volatile phase (overhead fraction). The less-volatile phasecontaining the rhodium catalyst and the salt of iodide (lithium iodide)as main components, and small amounts of methyl iodide, water and aceticacid was sent back to the reaction step from the bottom of thecatalyst-separating column. On the other hand, the higher-volatile phasecontaining, along with acetic acid, methyl acetate, methyl iodide andwater was distilled off as a distillate from the overhead of thecatalyst-separating column. The distillate contained 33.77% by weight ofmethyl iodide, 3.58% by weight of methyl acetate, 7.60% by weight ofwater, 0.01% by weight of propionic acid, 0.01% by weight ofacetaldehyde, and acetic acid as the rest.

(3) Purification Step

The overhead fraction (crude mixture) distilled from the overhead in theseparation step (2) of the higher bp catalyst component was fed to theoverhead of a distillation column (higher bp component-separationcolumn) (theoretical plate number of 12, operation pressure of 196 kPaas overhead pressure) at a rate of 1,200 g/h. Incidentally, the refluxof the higher bp component-separation column was not necessary becausethe above fraction (distillate) was fed to the overhead of the column.The bottom solution was withdrawn from the bottom of the column at abottom rate of 0.7 g/h. The bottom solution contained 2.56% by weight ofpropionic acid, and acetic acid as the rest.

The overhead fraction distilled from the overhead of the higher bpcomponent-separation column was supplied to the 17th plate from the topof a distillation column (carboxylic acid-separating column)(theoretical plate number of 38, operation pressure of 98 kPa asoverhead pressure) at a rate of 1199.3 g/h. The reflux ratio of thecarboxylic acid-separating column was 2.2, and acetic acid as a finishedproduct was obtained from the bottom of the column at a bottom rate of625 g/h. The bottom solution obtained from the column bottom contained300 ppm of water, 160 ppm of propionic acid, and acetic acid as therest.

The overhead fraction distilled from the overhead of the carboxylicacid-separating column was supplied to the 9th plate from the top of analdehyde-separating column (theoretical plate number of 18, operationpressure of 196 kPa as overhead pressure) at a rate of 574.3 g/h. Thereflux ratio of the aldehyde-separating column was 200, and the bottomsolution was obtained from the bottom of the column at a bottom rate of573.3 g/h. The bottom solution from the column bottom contained 70.5% byweight of methyl iodide, 7.5% by weight of methyl acetate, 16% by weightof water, and acetic acid as the rest.

The vapor amount to be used for heating from the higher bpcomponent-separation column to the aldehyde-separating column was 744 grelative to 1,000 g of acetic acid as a finished product.

Example 2

(1) Purification Step

The overhead fraction (crude mixture) which was obtained in theseparation step (2) of the higher bp catalyst component in Example 1 wassupplied to the 9th plate from the top of a first distillation column(lower bp component-separation column) (theoretical plate number of 10,operation pressure of 294 kPa as overhead pressure) at a rate of 1,200g/h. The reflux ratio of the lower bp component-separation column was1592, and the distillate was distilled off from the overhead of thecolumn at a distillation rate of 0.6 g/h. The resultant overheadfraction contained 20% by weight of acetaldehyde, 3% by weight of water,and methyl iodide as the rest.

The bottom solution which was withdrawn from the bottom of the lower bpcomponent-separation column was fed to the overhead of a seconddistillation column (higher bp component-separation column) (theoreticalplate number of 14, operation pressure in the distillation column of 101kPa as overhead pressure) at a rate of 119.4 g/h. Incidentally, thereflux of the higher bp component-separation column was not necessarybecause the bottom solution was fed to the overhead of the column. Thebottom solution was withdrawn from the column bottom of the higher bpcomponent-separation column at a bottom rate of 0.6 g/h. The bottomsolution from the column bottom contained 4.6% by weight of propionicacid, and acetic acid as the rest.

The overhead fraction distilled from the overhead of the higher bpcomponent-separation column was supplied to the 15th plate from the topof a third distillation column (carboxylic acid-separating column)(theoretical plate number of 40, operation pressure of 101 kPa asoverhead pressure) at a rate of 1198.8 g/h. The reflux ratio of thecarboxylic acid-separating column was 2.09, and acetic acid as afinished product was obtained from the bottom of the column at a bottomrate of 625 g/h. The obtained bottom solution contained 300 ppm ofwater, 148 ppm of propionic acid, and acetic acid as the rest.

In the carboxylic acid-separating column, the overhead fractiondistilled from the overhead of the column contained 70.5% by weight ofmethyl iodide, 7.5% by weight of methyl acetate, 16% by weight of water,and acetic acid as the rest.

The vapor amount to be used for heating from the lower bpcomponent-separation column to the carboxylic acid-separating column was884 g relative to 1,000 g of acetic acid as a finished product.

Example 3

(1) Purification Step

The overhead fraction (crude mixture) distilled from the overhead in theseparation step (2) of the higher bp catalyst component in Example 1 wassupplied to the 22nd plate from the top of a first distillation column(lower bp component-separation column) (theoretical plate number of 40,operation pressure of 101 kPa as overhead pressure) at a rate of 1200g/h. The reflux ratio of the lower bp component-separation column was1.37, and the bottom solution was withdrawn from the column bottom at abottom rate of 631.1 g/h. The bottom solution contained 0.9% by weightof water, 0.02% by weight of propionic acid, and acetic acid as therest.

The bottom solution which was withdrawn from the column bottom of thelower bp component-separation column was supplied to the second platefrom the top of a second distillation column (higher bpcomponent-separation column) (theoretical plate number of 27, operationpressure of 98 kPa as overhead pressure) at a rate of 631.1 g/h. Thereflux ratio of the higher bp component-separation column was 1, and thebottom solution was withdrawn from the column bottom at a bottom rate of0.3 g/h. The bottom solution from the column bottom contained 2.1% byweight of propionic acid, and acetic acid as the rest.

The distillate from the overhead of the higher bp component-separationcolumn was supplied to the 12th plate from the top of a thirddistillation column (carboxylic acid-separating column) (theoreticalplate number of 20, operation pressure of 98 kPa as overhead pressure)at a rate of 630.8 g/h. The reflux ratio of the carboxylicacid-separating column was 62.4, and acetic acid as a finished productwas obtained from the column bottom at a bottom rate of 625 g/h. Theobtained bottom solution contained 300 ppm of water, 152 ppm ofpropionic acid, and acetic acid as the rest.

The overhead fraction distilled from the overhead of the lower bpcomponent-separation column was further supplied to the 40th plate fromthe top of a forth distillation column (aldehyde-separating column)(theoretical plate number of 40, operation pressure of 196 kPa asoverhead pressure) at a rate of 568.9 g/h. The reflux ratio of thealdehyde-separating column was 400, and the bottom solution was obtainedfrom the column bottom at a bottom rate of 568.3 g/h. The obtainedbottom solution contained 71.5% by weight of methyl iodide, 7.6% byweight of methyl acetate, 16% by weight of water, and acetic acid as therest.

The vapor amount to be used for heating from the first distillationcolumn (lower bp component-separation column) to the forth distillationcolumn (aldehyde-separating column) was 1078 g relative to 1000 g ofacetic acid as a finished product.

Assuming that the equipment expenses of Example 2 were 1, those ofExample 3 were 3.8.

Comparative Example 1

Based on the flow diagram shown in FIG. 4, acetic acid was purified.

(1) Carbonylation Reaction

A rhodium catalyst, lithium iodide, methyl iodide, and water weresupplied to a reactor 63 at prescribed amounts so that the concentrationof the rhodium catalyst, that of lithium iodide, that of methyl iodide,and that of water were 400 ppm, 0.5 mol/L, 14% by weight, and 8% byweight in a mixture (liquid-phase system), respectively. The reactionwas carried out at 187° C. with feeding carbon monoxide and methanolthrough feed lines 61 and 62, respectively, to the reactor 63continuously to produce acetic acid.

(2) Separation Step of Higher Bp Catalyst Component

The reaction mixture (or crude reaction solution) obtained in thecarbonylation reaction (1) was fed to a distillation column(catalyst-separating column) 65 (temperature of 132° C., pressure of 252kPa) through a feed line 64, and was separated into a less-volatilephase (bottom fraction) and a higher-volatile phase (overhead fraction).The less-volatile phase containing the rhodium catalyst and the salt ofiodide (lithium iodide) as main components, and small amounts of methyliodide, water and acetic acid was sent back to the reaction system 63through a recycle line 67 from the bottom of the catalyst-separatingcolumn. On the other hand, the higher-volatile phase containing, alongwith acetic acid, methyl acetate, methyl iodide and water was distilledoff as a distillate from the overhead of the catalyst-separating column.The distillate contained 33.77% by weight of methyl iodide, 3.58% byweight of methyl acetate, 7.60% by weight of water, 0.01% by weight ofpropionic acid, 0.01% by weight of acetaldehyde, and acetic acid as therest.

(3) Purification Step

The overhead fraction (crude mixture) distilled from the overhead in theseparation step (2) of the higher bp catalyst component was supplied tothe 12th plate from the top of a first distillation column 68(theoretical plate number of 12, operation pressure of 235.2 kpa asoverhead pressure) through a feed line 66 at a rate of 1200 g/h. Thereflux ratio of the higher bp component-separation column 68 was 0.87,the bottom solution was withdrawn from the column bottom through abottom line 71 at a bottom rate of 12 g/h, and the overhead fraction wasremoved from the overhead through a distillation line 69. Moreover, aside-cut solution was withdrawn from the 10th plate from the top of thefirst distillation column at a discharge amount of 667 g/h. The bottomsolution contained 0.02% by weight of methyl acetate, 1.64% by weight ofwater, 0.05% by weight of propionic acid, and acetic acid as the rest.The side-cut solution contained 1.3% by weight of methyl iodide, 4.9% byweight of water, 0.017% by weight of propionic acid, and acetic acid asthe rest.

The side-cut solution of the first distillation column was supplied tothe third plate from the top of a second distillation column 72(theoretical plate number of 19, operation pressure of 274.4 kPa asoverhead pressure) through a feed line 70 at a rate of 667 g/h. Thereflux ratio of the second distillation column 72 was 8, the overheadfraction was separated through a distillation line 73 from the overheadof the column, and the bottom solution was obtained from the columnbottom at a bottom rate of 600 g/h. The obtained bottom solutioncontained 0.6% by weight of water, 0.017% by weight of propionic acid,and acetic acid as the rest.

The bottom solution obtained from the column bottom of the seconddistillation column was supplied to the 7th plate from the top of athird distillation column 75 (theoretical plate number of 16, operationpressure of 215.6 kPa as overhead pressure) through a feed line 74 at arate of 600 g/h. The reflux ratio of the third distillation column 75was 5, the bottom fraction was separated from the column bottom througha bottom line 77, and the distillate was obtained from the overhead ofthe column at a distillation rate of 599.46 g/h. The obtained distillatecontained 0.6% by weight of water, 0.015% by weight of propionic acid,and acetic acid as the rest.

The overhead fraction distilled from the overhead of the thirddistillation column was supplied to the 12th plate from the top of aforth distillation column 78 (theoretical plate number of 22, operationpressure of 98 kPa as overhead pressure) through a feed line 76 at arate of 599.46 g/h. The reflux ratio of the forth distillation column 78was 45. Along with the distillate was distilled off from the overhead ofthe column through a distillation line 79 at a distillation rate of 4.4g/h, a side-cut solution was withdrawn from the 22nd plate from the topof the distillation column through an extraction line 80 at anextraction rate of 595 g/h to obtain acetic acid as a finished product.Moreover, the bottom fraction obtained from the column bottom wasremoved through a bottom line 81. The distillate obtained from theoverhead of the column contained 78.4% by weight of water, and aceticacid the rest. Moreover, the side-cut solution contained 300 ppm ofwater, 151 ppm of propionic acid, and acetic acid as the rest.

The vapor amount to be used for heating from the first distillationcolumn to the forth distillation column was 3296 g relative to 1000 g ofacetic acid as a finished product.

Assuming that the equipment expenses of Example 1 were 1, those ofComparative Example 1 were 2.2. Moreover, assuming that the equipmentexpenses of Example 2 were 1, those of Comparative Example 1 were 8.3.

Comparative Example 2

(1) Purification Step

The overhead fraction (crude mixture) distilled from the overhead in theseparation step (2) of the higher bp catalyst component in ComparativeExample 1 was supplied to the 20th plate from the top of a firstdistillation column (theoretical plate number of 20, operation pressureof 235.2 kPa as overhead pressure) at a rate of 1200 g/h. The refluxratio of the first distillation column was 0.65, and the bottom solutionwas withdrawn from the column bottom at a bottom rate of 6 g/h.Moreover, a side-cut solution was withdrawn from the 19th plate from thetop of the distillation column at a discharge amount of 667 g/h. Thebottom solution contained 0.01% by weight of methyl iodide, 0.02% byweight of methyl acetate, 1.7% by weight of water, 0.04% by weight ofpropionic acid, and acetic acid as the rest. The side-cut solutioncontained 1.5% by weight of methyl iodide, 3.6% by weight of water,0.018% by weight of propionic acid, and acetic acid as the rest.

The side-cut solution of the first distillation column was supplied tothe third plate from the top of a second distillation column(theoretical plate number of 42, operation pressure of 176 kPa asoverhead pressure) at a rate of 667 g/h. The reflux ratio of the seconddistillation column was 7, and the bottom solution was obtained from thecolumn bottom at a bottom rate of 600 g/h. The bottom solution contained0.29% by weight of water, 0.016% by weight of propionic acid, and aceticacid as the rest.

The bottom solution obtained from the column bottom of the seconddistillation column was supplied to the 17th plate from the top of athird distillation column (theoretical plate number of 30, operationpressure of 215.6 kPa as overhead pressure) at a rate of 600 g/h. Thereflux ratio of the third distillation column was 5, and the distillatewas obtained from the overhead of the column at a distillation rate of599.46 g/h. The distillate contained 0.3% by weight of water, 0.018% byweight of propionic acid, and acetic acid as the rest.

The overhead fraction distilled from the overhead of the thirddistillation column was supplied to the second plate from the top of aforth distillation column (theoretical plate number of 22, operationpressure of 98 kPa as overhead pressure) at a rate of 599.46 g/h. Thereflux ratio of the forth distillation column was 66, and the distillatewas obtained from the overhead of the column at a distillation rate of4.4 g/h. Moreover, a side-cut solution was withdrawn from the 22nd platefrom the top of the forth distillation column at a discharge amount of595 g/h to obtain acetic acid as a finished product. The distillate fromthe overhead of the column contained 37.0% by weight of water, andacetic acid as the rest. The side-cut solution contained 300 ppm ofwater, 285 ppm of propionic acid, and acetic acid as the rest.

The vapor amount to be used for heating from the first distillationcolumn to the forth distillation column was 2215 g relative to 1000 g ofacetic acid as a finished product.

Assuming that the equipment expenses of Example 1 were 1, those ofComparative Example 2 were 1.6. Moreover, assuming that the equipmentexpenses of Example 2 were 1, those of Comparative Example 2 were 5.9.

1. A process for producing a carboxylic acid comprising allowing analcohol having “^(n)” carbon atom(s) or a derivative thereof to reactwith carbon monoxide continuously in the presence of a catalytic system,and purifying the resultant reaction mixture to give a purifiedcarboxylic acid having “n+1” carbon atoms, wherein a higher bp catalystcomponent is separated from the reaction mixture to give a crude mixturecontaining at least a carboxylic acid having “n+2” carbon atoms, acarboxylic acid having “n+1” carbon atoms, an ester of the carboxylicacid having “n+1” carbon atoms with the alcohol, and water; the crudemixture is fed to a higher bp component-separation column, and isseparated into a bottom fraction and an overhead fraction, the bottomfraction contains at least the carboxylic acid having “n+2” carbonatoms, and the overhead fraction contains at least the carboxylic acidhaving “n+1” carbon atoms, the ester of the carboxylic acid having “n+1”carbon atoms with the alcohol, and water; and the overhead fraction fromthe higher bp component-separation column is separated by a carboxylicacid-separating column into a bottom fraction and an overhead fraction,the bottom fraction contains the carboxylic acid having “n+1” carbonatoms, and the overhead fraction contains at least the ester and water.2. A process according to claim 1, wherein the reaction mixture containswater in a proportion of not more than 20% by weight.
 3. A processaccording to claim 1, wherein the crude mixture further contains analdehyde having “n+1” carbon atoms, and the crude mixture is fed to thehigher bp component-separation column.
 4. A process according to claim1, wherein the crude mixture containing the carboxylic acid having “n+2”carbon atoms, an aldehyde having “n+1” carbon atoms, the carboxylic acidhaving “n+1” carbon atoms, the ester of the carboxylic acid having “n+1”carbon atoms with the alcohol and water is fed to the higher bpcomponent-separation column, and is separated into the bottom fractionand the overhead fraction, the bottom fraction contains the carboxylicacid having “n+2” carbon atoms, and the overhead fraction contains thealdehyde having “n+1” carbon atoms, the carboxylic acid having “n+1”carbon atoms, the ester of the carboxylic acid having “n+1” carbon atomswith the alcohol, and water; the overhead fraction from the higher bpcomponent-separation column is separated by the carboxylicacid-separating column into the bottom fraction and the overheadfraction, the bottom fraction contains the carboxylic acid having “n+1”carbon atoms, and the overhead fraction contains at least the aldehyde,the ester and water; the overhead fraction from the carboxylicacid-separating column is separated by an aldehyde-separating columninto an overhead fraction and a bottom fraction, the overhead fractioncontains the aldehyde, and the bottom fraction contains at least theester and water; and the bottom fraction from the aldehyde-separatingcolumn is recycled to the reaction system.
 5. A process according toclaim 4, wherein the catalytic system comprises a catalyst containing ametal element of the Group 8 of the Periodic Table of Elements, analkali metal halide, and an alkyl halide; distillation in the carboxylicacid-separating column is carried out in the presence of the ester ofthe carboxylic acid having “n+1” carbon atoms with the alcohol, thealkyl halide and water for separating the bottom fraction from theoverhead fraction, the bottom fraction contains the carboxylic acidhaving “n+1” carbon atoms, and the overhead fraction contains water, thealkyl halide and the ester; the overhead fraction from the carboxylicacid-separating column is separated by the aldehyde-separating columninto the overhead fraction and the bottom fraction, the overheadfraction contains the aldehyde, and the bottom fraction contains water,the alkyl halide and the ester; and the bottom fraction from thealdehyde-separating column is recycled to the reaction system.
 6. Aprocess according to claim 1, wherein the crude mixture in which atleast an aldehyde having “n+1” carbon atoms has been removed is fed tothe higher bp component-separation column.
 7. A process according toclaim 1, wherein the higher bp catalyst component is separated from thereaction mixture to give a crude mixture, and the resultant crudemixture is fed to a lower bp component-separation column, and isseparated into the overhead fraction and the bottom fraction, theoverhead fraction contains at least an aldehyde having “n” carbonatom(s), and the bottom fraction contains at least the carboxylic acidhaving “n+2” carbon atoms; the bottom fraction from the lower bpcomponent-separation column is separated by the higher bpcomponent-separation column into the bottom fraction and the overheadfraction, the bottom fraction contains the carboxylic acid having “n+2”carbon atoms, and the overhead fraction contains at least the carboxylicacid having “n+1” carbon atoms, the ester of the carboxylic acid having“n+1” carbon atoms with the alcohol, and water; and the overheadfraction from the higher bp component-separation column is separated bythe carboxylic acid-separating column into the bottom fractioncontaining the carboxylic acid having “n+1” carbon atoms and theoverhead fraction containing at least the ester and water.
 8. A processaccording to claim 7, wherein the catalytic system comprises a catalystcontaining a metal element of the Group 8 of the Periodic Table ofElements, an alkali metal halide, and an alkyl halide; and distillationin the carboxylic acid-separating column is carried out in the presenceof the ester, the alkyl halide and water to give the bottom fractioncontaining the carboxylic acid having “n+1” carbon atoms, and theoverhead fraction containing at least the ester, the alkyl halide andwater.
 9. A process according to claim 7 or 8, wherein the overheadfraction separated by the carboxylic acid-separating column is recycledto the reaction system.
 10. A process according to claim 7, wherein theoverhead fraction separated by the lower bp component-separation columnis further fed to an aldehyde-separating column to separate an overheadfraction containing an aldehyde having “n+1” carbon atoms to give abottom fraction containing at least the ester and water; and the bottomfraction is recycled to the reaction system.
 11. A process according toany one of claims 1, 4 and 7, wherein distillation in the carboxylicacid-separating column is carried out in the presence of at least theester and water to give the bottom fraction containing the carboxylicacid having “n+1” carboxylic acid, and the overhead fraction.
 12. Aprocess according to claim 1 which comprises allowing at least onemember selected from the group consisting of methanol, methyl acetateand dimethyl ether to react with carbon monoxide continuously in thepresence of the catalytic system, and purifying the resultant reactionmixture to produce a purified acetic acid, wherein the higher bpcatalyst component is separated from the reaction mixture to give thecrude mixture; the crude mixture is fed to the higher bpcomponent-separation column, and is separated into the bottom fractionand the overhead fraction, the bottom fraction contains at leastpropionic acid, and the overhead fraction contains at least acetic acid,methyl acetate and water; and the overhead fraction from the higher bpcomponent-separation column is separated by the carboxylicacid-separating column into the bottom fraction and the overheadfraction, the bottom fraction contains said acetic acid, and theoverhead fraction contains at least said methyl acetate and water.
 13. Aprocess according to claim 12, wherein the catalytic system comprises acatalyst containing a rhodium catalyst, an alkali metal iodide andmethyl iodide; the crude mixture is separated by the higher bpcomponent-separation column into the bottom fraction and the overheadfraction, the bottom fraction contains at least propionic acid, and theoverhead fraction contains acetic acid, methyl acetate, methyl iodideand water; and the overhead fraction from the higher bpcomponent-separation column is distilled by the carboxylicacid-separating column in the presence of said methyl acetate and methyliodide, and is separated into the bottom fraction and the overheadfraction, the bottom fraction contains said acetic acid, and theoverhead fraction contains said methyl acetate, methyl iodide and water.14. A system for producing a carboxylic acid which comprises a reactionsystem for allowing an alcohol having “n” carbon atom(s) or a derivativethereof to react with carbon monoxide continuously in the presence of acatalytic system, a catalyst-separating column for separating a higherbp catalyst component from a reaction mixture generated in the reactionsystem, a higher bp component-separation column for separating a crudemixture obtained by a separation in the catalyst-separating column andcontaining at least a carboxylic acid having “n+2” carbon atoms, acarboxylic acid having “n+1” carbon atoms, an ester of the carboxylicacid having “n+1” carbon atoms with the alcohol, and water, into abottom fraction and an overhead fraction, wherein the bottom fractioncontains at least the carboxylic acid having “n+2” carbon atoms, and theoverhead fraction contains at least the carboxylic acid having “n+1”carbon atoms, the ester of the carboxylic acid having “n+1” carbon atomswith the alcohol, and water, and a carboxylic acid-separating column forseparating the overhead fraction separated by the higher bpcomponent-separation column into a bottom fraction and an overheadfraction, wherein the bottom fraction contains the carboxylic acidhaving “n+1” carbon atoms, and the overhead fraction contains at leastthe ester and water.